Knile Davis helped his draft stock during the combine with an impressive 40 time.

It's only fitting that, in a draft where most experts have only one running back being selected in the first round, Football Outsiders' Speed Score champion from the 2013 NFL combine is a third-day prospect at best -- or at least he was until Sunday. After running the 40-yard dash in an official time of 4.37 seconds and weighing in at 227 pounds, Arkansas running back Knile Davis blew away the competition with a Speed Score of 124.5.

Introduced on ESPN Insider back in 2008, Speed Score is Football Outsiders' metric for evaluating running back prospects. It's built on the simple idea that, because smaller backs tend to run faster than larger backs, we should be more impressed by a 4.5-second 40-yard dash from a 220-pound back than the same clock reading from a 170-pound back. As such, Speed Score incorporates a back's official time in the 40-yard dash with his weight to produce a measure of his speed given his size using the formula, (Weight * 200)/(40 time^4). As a guide, we consider a 100 Speed Score as average, with anything below 80 serving as a giant red flag, and anything above 120 serving as a giant neon sign.

To put this in context of our previously discussed prospect: Knile Davis' 124.5 Speed Score this year is off-the-charts good.

In fact, it's the second-highest in our database, which goes back to 1999. Successful NFL running backs since 2008 who broke the 120-point threshold include Darren McFadden (120.1), Chris Johnson (121.9).....

HERE ARE THE RESULTS OF ALL THE RB'S THIS YEAR... too bad Eddie Lacy wasn't on here...

I really like Christine Michael.
The thing is with all these RBs is that injuries are so damn common.... why I would almost always rather take a RB in rds 5-7 or UFA than use a 1st or 2nd round pick on one.

Brilliant letting one of Scott Pioli's henchmen have his own team to ruin. One of the premier GM jobs in the NFL and it gets handed to a stupid **** who makes three facepalm moves for every good one. Awesome. Just like handing a new Mercedes to a 16 year old girl who's already been in three wrecks.

Like I said in the combine thread. Christine Michael looked really good, but he overslept and missed two team interviews. So I don't think he helped his stock at all during the combine, if anything he hurt it. Just my opinion.

Sucks now though. Davis was a guy I've wanted for a long time. Viewed him as a sleeper in the 5-7 round range. Now, forget that.

If I remember enough of my college statistics, a correlation of r=.45 would be an r-squared=.20, meaning that "speedscore" accounts for 20% of the variance in the sample. (Somebody correct me if I've made any errors here. I'm rusty.)

So, it's not the be-all and end-all, but it certainly has value (assuming that the correlation still holds -- the original article was published 5 years ago). This shouldn't be surprising. The math just means "all else being equal, big and fast guys are better."

I always enjoy metrics like this. Some people get carried away with them, assuming that a single metric is the end-all be-all, but all it does is tell a side of the story. This formula just breaks down the straightline speed to size ratio of a runningback and gives a number accordingly. It's not a ranking of how good a runningback is because this formula doesn't factor in so many things that runningbacks do, it's simply a rating of how physically prototypical a runningback is based on just a couple of numbers: 40 time and weight.

That's why the 160-pound guy with the crazy 40 time is still below average, by the way. Being so small you might get carried away in the wind hurts you more in this formula than being able to run fast helps you. Runningbacks need to be able to take a hit, after all.

I like the speed score metric, but it's pretty clear that it's not the be-all and end-all since there's lots of other things that you want to take into account that have nothing to do with the speed score (e.g. vision, balance, patience, blitz pickup, hands, character) and the speed score isn't weaker because it doesn't reflect those things, since it doesn't even try to.

The simple question is "do you want a RB who hits the guy who's trying to tackle him as hard as possible?" and the answer is "yes, yes you do." There is no context in which it's worse for a running back to be capable of delivering less of a blow to would-be-tacklers. There's two ways to hit hard: be big, or be fast. Linear Momentum is m*v and Kinetic Energy is .5m(v)^2.

I think there are some things fundamentally wrong with this formula/concept.

First, I think a better measure is mass/time, not mass/time^4

The reason is because momentum is mass*velocity, and the velocity of a player is proportional to 1/time. Momentum is an important measure for breaking tackles.

Second, most broken tackles occur close to the LOS so the 40-time is not a a good measure of time. The 20 yd shuttle or 10 yd split may be better.

Speed is still important for busting out long runs, and quickness is also an important measure for elusiveness. So the 40-time, shuttle/cone drills, and mass/time are all independent measures of a different abilities of a player.

I think the best thing to do would be to take a weighted average of each ability. Mass/time would be an important measure of momentum/break tackles, 40-time indicates pure speed, shuttle/ten yd split measures elusiveness. Similar to what is done for madden ratings